This invention generally relates to a surface treating solution for zinc, copper, nickel, silver, iron, cadmium, aluminum, magnesium, and their alloys, a method of applying surface coatings, and coated metallic materials. The invention specifically relates to a surface treating solution and a treating method for forming protective coating films on zinc- and zinc alloy-coated iron parts, and surface treated metallic materials.
There are various films as protective coating films on zinc, copper, nickel, silver, iron, cadmium, aluminum, magnesium, and their alloys. However, no such film that corresponds to any according to the present invention has been found yet, and this invention provides newly discovered coating films. The most common of corrosion-preventive methods in use for iron articles and parts is coating with zinc or zinc alloy (hereinafter called "galvanizing"). Galvanized iron articles and parts, if used as they are, would readily form a zinc white rust. To avoid this, they are usually provided with a protective coating film over the galvanized surface. Protective coating films that are conventionally used on zinc coat are formed by phosphate and chromate treatments. Chromate treatment is divided into three types; electrolytic, coating, and reaction type chromate treatments. These treatments are applicable not only to zinc but also to aluminum, cadmium, magnesium, and their alloys.
Phosphate treatment is a process, as taught in Patent Application Kokai No. 3-107469, which comprises immersing an object to be coated in a treating solution which consists essentially of zinc ion and phosphate ion as film-forming components and fluoride ion or complex fluoride ion as an etching or film-densifying agent, heated to 40 to 50.degree. C. or up to about 75.degree. C., thereby forming a coating film on the object, water washing, and then drying the coated object. The surface of the coating film thus obtained is very rough with the needle crystals of zinc phosphate piled up. This surface condition helps improve the adhesion of paint and enhance the corrosion resistance of the painted surface, achieving the dual purpose of the film. However, the film before painting is seriously short of rust-inhibiting capacity (corrosion resistance). Moreover, the surface as treated looks dull gray to grayish white and lacks ornamental effect. Since the treated surface is not aesthetically attractive, it is not suited for articles that are partly or wholly unpainted. Phosphate films essentially contain fluoride ion or complex fluoride ion without which they cannot be formed, but either ion is strongly corrosive and comes in the list of substances under emission control. High treating temperature, and extra equipment and cost for heating are additional disadvantages.
On the other hand, chromate film before painting is superior to phosphate film in corrosion resistance. However, chromate treatment has recently caused growing concern, because of the adverse effects upon the human beings and the environments of the treating solution that necessarily uses poisonous hexavalent chromium and also because of the chromium itself that dissolves out of the treated articles. This is an insurmountable problem since chromate film essentially depends on the hexavalent chromium for its corrosion resistance. Another knotty problem that is always associated with electrolytic chromate treatment in which a chromate film is formed by electrolysis is the problem of throwing power, especially with workpieces of components naturally of far intricate configurations than steel sheets. In addition, the mist of chromic acid that results from the electrolysis can cause more serious environmental pollution than other known processes. Coating type chromate treatment comprises applying an acidic aqueous solution essentially containing chromic acid to a metallic surface and, without water washing, drying the coated surface with heat. Like electrolytic chromating, the coating type is not suited for workpieces of complex configurations. Moreover, the process has its limitation on the uniformity of coating film thickness. This combines with the omission of water washing to make the treated surface as uneven as with the phosphate film. The coated film, therefore, is unable to satisfy the users' aesthetic requirements when used alone and, like the phosphate film, it is commonly employed as a mere undercoat. Reaction type chromate treatment, by contrast, is often adopted as finish coating as well as undercoating because of the uniform appearance and stable corrosion resistance of the coating film. It has the unsettled pollution problem of hexavalent chromium, however.
The present invention has for its object to form protective coating films which combines a uniform, good appearance and corrosion resistance on the surfaces of zinc, copper, nickel, silver, iron, cadmium, aluminum, magnesium, and their alloys, without using noxious hexavalent chromium or strongly corrosive fluorine compounds. A particularly important object is to provide protective coating films on galvanized iron articles other than steel sheets, for which coating type treatment on an industrial scale has hitherto been practically difficult.